Many organic-rich mudstone deposits have been
interpreted in the context of water column stratification. Here, Könitzer et al. examine temporal
variations of, and controls on, the abundance and type of organic matter in late
Paleozoic organic-rich mudstones from an epicontinental marine basin. Detailed microtextural
analysis with data on total organic carbon (TOC) and bulk carbon isotope
composition of organic material (δ13Corg) suggest alternative mechanisms for deposition of Lower
Carboniferous succession. The data are interpreted to represent changes in
productivity, variations in the delivery of siliciclastics and terrestrial
organic matter. The abundance of organic matter stored in these extensive
marine basins influenced the carbon cycle during this icehouse period in Earth
history and the suitability of Late Mississippian mudstone successions of the U.K.
as prospects for shale gas.
Questions of how shorefaces
respond to relative rises in sea level are of paramount importance for
predicting morphosedimentary response to rising global sea level. In this
study, Gzam and others examine a
suite of beach ridges along the Tunisian coast to better understand their
genesis and dynamics. The results, which integrate petrographic and facies
analysis, transverse profile surveys, and field observations, suggest that
these progradational, Mid-Holocene to recent beach ridges formed during periods
of relative highs in sea level. The data also reveal that the modern beach
ridge is composed mainly of shell debris, whereas the Holocene succession
consists of siliciclastic sand, suggesting that beach ridges were nourished
from two distinctive sediment sources.
Many deep-water fans include
intimately associated channels and lobes, formed by dynamic conditions of flow,
sedimentation and erosion, and gradient. Fernandez et al. describe a
series of large-scale experiments on non-channelized turbidity currents that
illustrate the evolution and complex stratigraphy of channel-lobe systems
developed updip and downdip of a break in slope. Data and analysis examined:
(i) velocity and suspended sediment concentration of the flows themselves; (ii)
time and spatial evolution of channel and lobe construction, and (iii) spatial
trends in grain-size distribution along the deposit. The results provide a
comparative picture of the gross structure of the fans, with information on
their surfaces, growth dynamics, and times of activity of the incised channels
and lobed features. Of particular note is the role that the break in slope
played in governing channel aggradation and lobe architecture over the deposit,
and ultimately controlling the dimension, geometry, and connectivity of the
deposits.
Recent discoveries
of hydrocarbons in pre-salt carbonate deposits of the South Atlantic have
generated significant interest in microbial influences on carbonate mineral
precipitation. Fifty years ago, Scholland Taft broke with conventional
wisdom (“…that inorganic processes of
tufa deposition at Mono Lake are so dominant that tufa formation by algae is
not significant”) and suggested the important role of algae on
mineralization. The careful field, slab, and petrographic observations were
interpreted to reflect the situation in which “algae initiate much of the precipitation and thereby fashion the
calcitic or aragonitic framework of lithoid tufa.”
